1. Statement of the Technical Field
The inventive arrangements relate generally to transmission line stubs, and more particularly for transmission line stubs that can be dynamically tuned.
2. Description of the Related Art
Transmission line stubs are commonly used in radio frequency (RF) circuits. A transmission line stub is sometimes said to be resonant at a particular frequency, meaning the line has impedance characteristics similar to a resonant circuit at that frequency. Accordingly, transmission line stubs are often referred to as tuned lines or resonant lines. It should be noted, however, that transmission line stub impedance characteristics are actually a function of voltage reflections, not circuit resonance.
On printed circuit boards or substrates, transmission line stubs are typically implemented by creating a line with at least one port at the input, and either an open circuit or short circuit to ground at the termination. On an open circuited transmission line stub, each point at an even number of quarter-wavelengths from the termination is at a position of voltage maxima and has a high impedance, while each point at an odd number or quarter wavelengths from the termination is at a position of voltage minimum and has a low impedance. Notably, the relative positions of voltage maxima and minima on a shorted-circuited transmission line stub are reversed in comparison to the positions of voltage maxima and minima on an open circuited transmission line stub.
The input impedance to an open or shorted transmission line stub is typically resistive when the length of the transmission line stub is an even or odd multiple of a quarter-wavelength of the operational frequency. That is, the input to the transmission line stub is at a position of voltage maxima or minima. When the input to the transmission line stub is at a position between the voltage maxima and minima points, the input impedance can have reactive components. Consequently, properly chosen transmission line stubs may be used to provide complex impedance characteristics.
Transmission line stubs in RF circuits are typically formed in one of three ways. One configuration known as microstrip, places the signal line on the top of a board surface. A second conductive layer, commonly referred to as a ground plane, is spaced apart from and below the signal line. A second type of configuration known as buried microstrip is similar except that the signal line is covered with a dielectric substrate material. In a third configuration known as stripline, the signal line is sandwiched between two electrically conductive (ground) planes. Other configurations, including waveguide stubs, are also known in the art.
The electrical characteristics of transmission line stubs generally cannot be modified once formed on an RF circuit board. This is not a problem where only a fixed frequency response is needed. The geometry of the transmission line can be readily designed and fabricated to achieve the proper characteristic impedance. When a variable frequency response is needed, however, use of a fixed length transmission line stub can be a problem.
A similar problem is encountered in RF circuit design with regard to optimization of circuit components for operation on different RF frequency bands. Line impedances and lengths that are optimized for a first RF frequency band may provide inferior performance when used for other bands, either due to impedance variations and/or variations in electrical length. Such limitations can limit the effective operational frequency range for a given RF system.